High efficient VCSELs require highly reflective semiconductor mirrors, typically realised as distributed Bragg reflectors (DBR). Doped DBRs show a higher absorption of the generated laser light than undoped DBRs. Therefore, for high efficient VCSEL designs the use of undoped DBRs is preferred.
R. C. Strijbos et al., “Current crowding in oxide-confined intracavity-contacted VCSELs”, Proceedings Symposium IEEE/LEOS Benelux Chapter, 2000, Delft, The Netherlands, pages 223 to 226 disclose such a top-emitting VCSEL with undoped DBRs. On both sides of the active region contact layers are formed to which a metallization is applied in order to electrically contact the contact layers from the top side.
High power VCSEL array designs, consisting of multiple single VCSEL light sources, need to take care of homogeneous carrier injection. This means that all single VCSELs in a VCSEL array typically get the same operation current and that a voltage drop over the electrical lines of the array must be avoided. In case of a state of the art top-emitting VCSEL array, a highly conductive substrate is used for the homogeneous n-current injection and a thick, usually plated, gold layer is used for homogeneous p-carrier-injection. This requires an electrically conducting (doped) bottom DBR, which then on the other hand has an increased absorption for the laser light and thus lowers the efficiency of the VCSELs.